1. Field of the Invention
This invention relates to a resin-coated carrier for two component type developers used when latent images are developed by electrophotography or electrostatic recording, a two component type developer having such a carrier, and a developing method making use of the carrier.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 (U.S. Pat. No. 3,666,363) and No. 43-24748 (U.S. Pat. No. 4,071,361) are conventionally known as electrophotography. In general, copies are obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a developer to form a visible image as a toner image, and transferring the toner image to a transfer medium such as paper when necessary, followed by fixing with heat and/or pressure.
The developer used here includes two component type developers comprised of toners and carriers, and one component type developers making use of toners alone as exemplified by magnetic toners. The two component type developers allot to the carrier the function to agitate, transfer and electrostatically charge the developer so as to be functionally separated as the developer. Hence, they have a good controllability, and are in wide use at present. In particular, developers making use of resin-coated carriers comprising a carrier core material whose surface has been coated with a resin can make resistance optimum, have an excellent charge controllability and can relatively easily be improved in qualities, such as environmental dependence or stability with time.
As core material particles, ferrite particles are widely used because of, e.g., light weight, excellent fluidity and superior controllability for magnetic properties. As developing methods, cascade development was used formerly, but magnetic brush development making use of a magnetic roll as a developer carrying member is prevalent at present. A developing apparatus employing this magnetic brush development usually has a developing sleeve which is a cylindrical developer carrying member provided internally with a magnetic roller comprising a magnetic body having a plurality of magnetic poles. On the surface of this developing sleeve, a magnetic carrier having a toner attracted thereto is carried is transported to a developing zone to make development. Also, in this magnetic brush development, it is common to apply an AC electric field to the developing bias in order to improve development efficiency.
In addition, in recent years, a technique has made progress in which, in the course of the formation of electrostatic latent images on a photosensitive member, a small-diameter laser beam is used to expose the photosensitive member. This has made the electrostatic latent images more minute. Concurrently therewith, toner particles and carrier particles are both being made smaller in diameter so that electrostatic latent images can faithfully be developed to achieve a higher image quality. In particular, it is often attempted to to decrease the average toner particle diameter to improve image quality.
Making the toner's average particle diameter smaller is an effective means for further improving image characteristics, in particular, graininess and character reproducibility, but has problems to be solved with regard to specific image quality items.
In the first place, the use of toner over a long period of time causes carrier contamination, i.e., "toner-spent", resulting in a lowering of electric charge that cause fog and toner scatter conspicuously. Such a phenomenon tends to be caused by making the toner's average particle diameter smaller. This phenomenon tends to be more pronounced when picture element units of electrostatic latent images are made minute.
Second, in instances where originals having a high image area percentage are used, there may be a time lag until the toner becomes uniformly charged when supplied in a large quantity, which is a phenomenon due to a decrease in fluidity caused by making the toner have a smaller particle diameter. Such a phenomenon, which causes faulty images, is pronounced especially when multi-color superimposed images are formed using a two component type developer, and must be prevented. This problem has long been discussed in studies on carrier resistance, but it has not yet been solved.
Triboelectric charging is made to occur by physical external force such as contact or collision of the toner with or against the carrier, and hence both the toner and the carrier may inevitably be damaged. For example, as for the toner, any external additive added to its particle surfaces may become buried in toner particles, or toner components may come off. As for the carrier, it may become contaminated by toner components inclusive of any external additive or, in the case of the resin-coated carriers, the carrier coat component may wear or break. Such damage makes it impossible to maintain the initial performances of developers with repetition of copying and causes ground fog, in-machine contamination, and variations of image density.
To solve these problems, it is attempted to use the carrier in a large quantity. Under existing circumstances, however, carriers having a sufficient durability have not been obtained.
In the two component type developing system, as disclosed in Japanese Patent Application Laid-Open No. 5-19632, a method is proposed in which the developing sleeve is made to have a large surface roughness to improve toner transport performance.
If, however, the developing sleeve merely has a large surface roughness, the sleeve surface may be scraped as a result of its friction with the developer or the toner may become buried in the uneven surface, resulting in a poor durability. If, on the other hand, the developing sleeve surface is made to merely have release properties in order to prevent it from contamination by toner, the surface tends to be so slippery as to have a poor transport performance, making it difficult for the developer to be stably fed to the developing zone. This may cause a local increase or decrease in toner concentration (i.e., toner-carrier mixing ratio) on the developing sleeve, tending to result in blurred images or non-uniform image density. If, on the other hand, the developing sleeve surface is formed of a hard metal, the coat material on the carrier particle surfaces tends to come off to accelerate carrier deterioration. Also, the use of a toner having a good fluidity may cause a decrease in frictional resistance to the developing sleeve, so that the developer may slip off and not be well transported and the developer may stagnate at the lower part of the developing sleeve, tending to cause a developer leak. Such a developer leak can not fundamentally be settled only by merely improving the mechanism of developing assemblies, thus there arises a problem that the transport performance attributable to the developing sleeve must further be improved.
Moreover, in recent years, there is an increasing commercial demand for copying machines achieve a higher minuteness and making images have a higher quality. In the present technical field, it is attempted to make toner particle diameter smaller so that color images can be formed in a high image quality. Making the particle diameters of toner smaller, however, results in an increase in the surface area per unit weight, tending to bring about a large charge quantity of the toner. This is accompanied with a possibility of the insufficiency of image density or the deterioration of running performance.
When copies of an original having a large image area percentage are continuously made on many copy sheets, sharp images with a good image quality can be obtained at the initial stage, but the edge effect with much fog may occur after copies have been made on several tens of thousands of sheets, resulting in images having poor gradation and sharpness. In this regard, the transport performance of a developer on a developing roller is very important.
Reports hitherto made for the purpose of maintaining a high image quality are exemplified by Japanese Patent Application Laid-Open No. 2-877, which discloses a toner containing toner particles with a size of 5 .mu.m or smaller in an amount of 17 to 60% by number. This shows a strong tendency to make a toner have a smaller particle diameter. In such a case, when originals requiring a large toner consumption as in photograph originals are continuously copied, the particle size distribution of toner may change if measures are taken only from the direction of toners, making it difficult to always obtain stable images.
Meanwhile, Japanese Patent Applications Laid-Open No. 51-3238, No. 58-144839 and No. 61-204646 suggest average particle diameter and particle size distribution of carriers. Of these, Japanese Patent Application Laid-Open No. 51-3238 makes reference to a rough particle size distribution. Also, Japanese Patent Application Laid-Open No. 58-23032 discloses a carrier comprising a porous material with many voids. Such a carrier, however, tends to cause toner-spent, and does not necessarily satisfy the running stability. Japanese Patent Application Laid-Open No. 8-95386 reports a carrier at a development position and the surface roughness of a developing sleeve. Taking such measures only, however, the carrier can not stably impart triboelectricity, and does not necessarily satisfy the running stability.
Japanese Patent Application Laid-Open No. 7-98521 reports the particle size distribution and specific surface area of a carrier. This, however, is still insufficiently adaptable to high speed copying.
Nowadays, copying machines are long expected to have the ability to continuously copy a graphic picture having an image area percentage of 20% or more and the ability to lessen the edge effect and retain the uniformity of image density of a copy on one sheet.
Japanese Patent Application Laid-Open No. 9-319161 discloses that, with regard to a carrier comprising a core material having thereon a resin coat layer formed of a matrix resin in which specific thermosetting fine resin particles have been dispersed and incorporated, the core material may have shape factors SF-1 of from 100 to 145 and SF-2 of from 100 to 120, whereby uniform coatings can be formed with ease at the time of resin coating, the distribution of electric charges in toner can be made narrow and also the toner can be kept from its impaction, so that the ability to impart triboelectricity to toner can be maintained with stability.
However, in the carrier disclosed in this publication, the particle size distribution of the carrier having the resin coat layer formed thereon is not controlled, and a further improvement should be made in respect of the prevention of carrier adhesion to and carrier scatter on the photosensitive member.
In Japanese Patent Application Laid-Open No. 10-39549, with regard to a magnetic coated carrier comprising magnetic carrier core particles containing metal oxide particles and whose particle surfaces have been coated with a resin composition, various physical properties of the magnetic carrier core particles and magnetic coated carrier are specified. In particular, it discloses that the magnetic coated carrier may have a number-average particle diameter of from 1 to 100 .mu.m and the distribution cumulative value of number distribution of particles not larger than 1/2-fold diameter of number-average particle diameter may be 20% by number or less, whereby the carrier adhesion can well be prevented, and also the magnetic coated carrier may have a shape factor SF-1 of from 100 to 130, whereby the developer can have a good fluidity and has a superior ability to impart triboelectricity to toner, the shape of magnetic brush can be uniform at development poles and images with a high image quality can be obtained.
However, in the carrier disclosed in this publication, any surface properties of the carrier core material are not taken into account, and there is room for further improvement in respect of developer transport performance and prevention of carrier scatter.
In conventional processes for producing carriers, it is prevalent to adjust the carrier resistance. More specifically, this is done to make the apparent resistance uniform by coating particles with resin in a large quantity in respect of a core material having a large surface unevenness, having a large specific surface area, and on the other hand by coating particles with resin in a small quantity in respect of a core material having a small surface unevenness, having a small specific surface area.
However, as discussed previously, there is an increasing commercial demand for achieving a higher minuteness and a higher image quality, and it is attempted to make toner particle diameter smaller and to merely make carriers have a small diameter for the purpose of an improvement of development efficiency. Thus, the situation is such that a deflection of materials is no longer tolerable, and it is sought to find a factor that holds the key of making the deflection of materials small. None of the previously disclosed copying machines can attain the image quality high enough to cope with the running. Under existing circumstances, it is difficult to achieve all the high image density, the high image quality and prevent fog and carrier adhesion.